Heretofore, a vehicular seatback has been known (for example, see Patent Reference 1) in which a damping control device is provided between a seatback frame and an active headrest. This vehicular seatback is specified such that a resonance frequency of the damping control device substantially matches a lateral (left-right) resonance frequency of the active headrest. Thus, lateral direction vibrations of the seatback are mitigated.    Patent Reference 1: Japanese Patent Application Laid-Open (JP-A) No. 2007-161102